Polyisoprene articles and process for making the same

ABSTRACT

The invention disclosed herein relates to an improved process for making elastomeric polyisoprene articles. In particular, the process of the invention is a system which produces synthetic polyisoprene articles exhibiting tensile strength properties similar to that of solvent-based processes using natural rubber latex. The process comprises an accelerator composition at the pre-cure stage comprising a dithiocarbamate, a thiazole and a guanidine compound. In a preferred embodiment, the accelerator composition comprises zinc diethyldithiocarbamate (ZDEC), zinc 2-mercaptobenzothiazole (ZMBT) and diphenyl guanidine (DPG), in conjunction with a stabilizer, such as sodium caseinate. The invention also includes an elastomeric polyisoprene product made by the process, such as a surgeon&#39;s glove.

RELATED APPLICATION DATA

[0001] This application is based on U.S. provisional patent applicationSerial No. 60/275,087 filed on March 12, 2001.

FIELD OF THE INVENTION

[0002] The invention relates to the field of elastomeric articles usedin the medical field. In particular, the invention relates toimprovements to the process of making elastomeric polyisoprene articlesfor medical applications.

BACKGROUND OF THE INVENTION

[0003] The manufacturing process for producing elastomeric articles fromnatural or synthetic rubber latex involves a curing step during whichcrosslinking or vulcanization through sulfur groups occurs between thepolymer units. Conventional processes for making elastomeric articlesfrom natural or synthetic latex typically involve preparing a latexdispersion or emulsion, dipping a former in the shape of the article tobe manufactured into the latex and curing the latex while on the former.Desirable properties of certain elastomeric articles such as tensilestrength are substantially affected by the cross-linking and curingstages of the manufacturing process.

[0004] The use of vulcanizing or sulfur cross-linking acceleratorcompounds in the manufacture of rubber articles is well-known.Conventional vulcanization accelerators include dithiocarbamates,thiazoles, guanidines, thioureas, amines, disulfides, thiurams,xanthates and sulfenamides. The use of vulcanization accelerators in themanufacture of polyisoprene rubber is disclosed in D'Sidocky et al.,U.S. Pat. No. 5,744,552 and Rauchfuss et al., U.S. Pat. No. 6,114,469.Certain fields in which elastomeric articles are needed, such as themedical field, utilize specific types of equipment and processingtechniques which accommodate the specific performance and regulatoryrequirements of the particular article produced.

[0005] The use of natural rubber latex in the manufacture of certainarticles such as medical gloves has been associated with disadvantageousproperties, such as allergic reactions believed by some to be caused bynatural proteins or allergens present within the natural rubber latexand the final product. Of increasing interest in the medical field,particularly in the field of gloves, are synthetic elastomeric productsand manufacturing processes which altogether reduce, or altogetheravoid, the likelihood of potential adverse reactions of the user orwearer.

[0006] Synthetic elastomeric polyisoprene articles such as gloves areknown and are of interest in the art as an alternative to the use ofnatural latex. Commercially available synthetic gloves include thoseelastomers composed of polychloroprene (neoprene), carboxylatedacrylonitrile butadiene (nitrile),styrene-isoprene-styrene/styrene-ethylene-butylene-styrene blockco-polymers, polyurethane, and polyisoprene. Polyisoprene is one of themost preferred polymers due to its chemical similarity to natural rubberas well as its physical properties such as feel, softness, modulus,elongation and tensile strength. One such polyisoprene glove iscommercially available from Maxxim Medical (Clearwater, Fla.).

[0007] A majority of glove manufacturing processes are water-baseddipping systems. It is known that solvent-based systems are possible forpolyisoprene, although such systems are poorly suited for themanufacture and molding of elastomeric articles for medicalapplications. One difficulty in the field of gloves, for example, is thedesign of processes and materials which will produce a thin elastomericarticle having desirable properties such as high tensile strength.Another disadvantage of solvent-based systems is solvent toxicity.Process and materials which would obviate or reduce the need for the useof toxic solvents while at the same time yielding a product havingdesirable properties for medical applications are thus still beingexplored.

[0008] Accordingly, there exists a need in the medical device field forimproved manufacturing processes for making synthetic elastomericarticles. Especially desirable would be processes which can producepolyisoprene articles, such as surgical gloves, which possess thedesirable properties found in the natural rubber counterpart, while atthe same time permitting economical and cost-effective manufacturing.

SUMMARY OF THE INVENTION

[0009] Applicants have discovered a three-part accelerator compositionfor sulfur cross-linkable polyisoprene latex which can be used withlatex in a process for making elastomeric articles having the desirableproperties (e.g., tensile strength) similar to that of natural rubberbut without the presence of natural rubber latex proteins and allergens.Another advantage is that the accelerator system is suitable for medicalapplications where thin molded elastomeric articles are required, suchas gloves. Furthermore, the accelerator composition and process of theinvention permits the use of a solvent-free, water-based process system,as opposed to a solvent based process system. The resultant article hasproperties similar to those produced using the solvent-based system.Accordingly, the use of solvents can be reduced or avoided and solventtoxicity can likewise be avoided using the invention.

[0010] Another advantage of the invention is that conventionalmanufacturing equipment and most readily-available materials can be usedin accordance with the invention to make the synthetic polyisopreneglove without the need for new or costly additional materials orequipment. Further, no complicated new process steps are required by theinvention and the invention can be readily incorporated into existingglove making processes and systems.

[0011] Another aspect of the invention is that the compounded (or readyto use) polyisoprene latex composition formulated in accordance with theinvention exhibits prolonged storage stability. For example, thepre-cure storage stability of the compounded polyisoprene latexcomposition (i.e., the time period prior to the use of the compoundedpolyisoprene latex composition in the dipping and curing stages) canextend up to about 8 days, in contrast to the typical current 3 to 5 daytime period. By extending storage life of the latex, the amount ofwasted latex can be significantly reduced and greater flexibility inscheduling manufacturing processes is permitted.

[0012] Yet another advantage is that the process of the invention allowsfor significantly reduced pre-cure process parameters (lower temperatureand shorter time periods than conventionally used) and lower dippingtemperatures in the manufacturing process. Accordingly, significant costand resource advantages are provided over conventional manufacturingpractices.

[0013] The invention provides for a process of making a syntheticelastomeric polyisoprene article comprising the steps of: a) preparing acompounded polyisoprene latex composition containing an acceleratorcomposition containing a dithiocarbamate, a thiazole and a guanidinecompound; b) dipping a former into said compounded polyisoprene latexcomposition; and c) curing said compounded polyisoprene composition onsaid former. Additionally, the initial pre-cure processing (i.e., priorto storage and article manufacture) can be performed at temperatures ofless than 35° C. and in time periods as short as ranging from about 90minutes (1.5 hours) to about 150 minutes (2.5 hours), preferably about120 minutes (2.0 hours). The compounded polyisoprene latex compositioncan be stored for periods up to about 8 days at ambient temperatures(ranging from about 15° C. to about 20° C.). Lower temperatures can beused for the latex dipping step as well

[0014] The invention also provides for a synthetic elastomericpolyisoprene article made by a process comprising the steps of: a)preparing a compounded polyisoprene latex composition comprising anaccelerator composition comprising a dithiocarbamate, a thiazole and aguanidine compound; b) pre-curing said compounded polyisoprene latexcomposition c) dipping a former into said compounded polyisoprene latexcomposition; and d) curing said compounded polyisoprene composition onsaid former. Elastomeric articles made by the process of the inventioncan exhibit tensile strengths of over 3000 psi (as measured inaccordance with ASTM D412) even after as much as 7 days of latex storageprior to use in the article manufacturing process.

[0015] The invention further provides for a synthetic polyisoprene latexcomposition comprising:

[0016] polyisoprene latex;

[0017] a dithiocarbamate compound;

[0018] a thiazole compound; and

[0019] a guanidine compound.

[0020] The invention also provides for an accelerator composition foruse in making elastomeric polyisoprene articles consisting essentiallyof:

[0021] a dithiocarbamate compound;

[0022] a thiazole compound;

[0023] a guanidine compound;

[0024] wherein the phr (parts per hundred) dry weight ratio of each ofthe dithiocarbamate; thiazole; and guanidine ranges from about 0.50 toabout 1.00 per 100.0 parts polyisoprene.

[0025] In a preferred embodiment, the accelerator composition compriseszinc diethylthiocarbamate (ZDEC), zinc 2-mercaptobenzothiazole (ZMBT)and diphenyl guanidine (DPG) and used in conjunction with a stabilizer.Preferably, the stabilizer is an alkali earth metal caseinate salt, suchas sodium caseinate.

DETAILED DESCRIPTION OF THE INVENTION

[0026] The accelerator composition of the invention can be used inconjunction with conventional equipment and materials otherwise known tobe used in the manufacture of elastomeric articles composed ofpolyisoprene. In general, the process begins with the preparation of thecompounded polyisoprene latex composition. The synthetic polyisoprenelatex is combined with the accelerator composition, a stabilizer, andadditional ingredients to prepare the polyisoprene latex composition inaccordance with the invention. The function of the accelerator is toincrease the rate of vulcanization, or the cross-linking of polyisopreneto enhance the curing properties of the latex during the curing stagesof the process. Prior to the dipping and curing steps, the compoundedlatex including the accelerator composition can be used immediately orstored for a period of time prior to its employment in the dippingprocess.

[0027] When the compounded polyisoprene latex composition is ready foruse or following storage, a former in the overall shape of the articleto be manufactured is first dipped into a coagulant composition to forma coagulant layer directly on the former. Next, the coagulant-coatedformer is dried and then dipped into the compounded polyisoprene latexcomposition.

[0028] The latex-covered former is then subjected to the curing step.The latex is cured directly onto the former at elevated temperaturesthereby producing an article in the shape of the former. Further stepsare typically performed as well, such as leaching with water, beadingthe cuff, and the like. These techniques are well-known in the art.Additional post-treatment processes and techniques steps are oftenperformed as well, such as lubrication and coating, halogenation (e.g.,chlorination), and sterilization.

[0029] A variety of elastomeric articles can be made in accordance withthe invention. Such elastomeric articles include, but are not limitedto, medical gloves, condoms, probe covers (e.g., for ultrasonic ortransducer probes), dental dams, finger cots, catheters, and the like.As the invention provides numerous advantages and benefits in a numberof ways, any form of elastomeric article which can be composed ofpolyisoprene can benefit from the use of the invention.

[0030] Polyisoprene latex is the major component of the pre-cure latexcomposition. Suitable polyisoprene latex which can be used is readilyavailable and can be obtained from a number of commercial sources,including but not limited to, Kraton™ Corporation, Houston, Tex.; ShellInternational Corporation, Houston, Tex.; Apex Medical Technologies,Inc. San Diego, Calif.; and Aqualast™ E0501 available from LordCorporation, Erie, Pa. In addition to polyisoprene, polyisopreneco-polymers and polyisoprene blends can be used as well. Polyisopreneco-polymers which can be used include any co-polymer having an isoprenemonomer unit and having sufficiently similar chemical structural andproperties of polyisoprene to exhibit the desirable properties of thepolyisoprene product when combined with the accelerator composition andmade according to the process of the invention. Suitable polyisopreneblends can include, but are not limited to: natural rubber latex;polydiene and its co-polymers, such as polybutadiene; substitutedpolydiene, such as polychloroprene; thermoplastic materials, such aspolyurethane; and the like.

[0031] The accelerator composition of the invention comprises at leastone dithiocarbamate, at least one thiazole, and at least one guanidinecompound. Preferably, the dithiocarbamate compound for use with theinvention is zinc diethyldithiocarbamate, also known as ZDEC or ZDC.Suitable ZDEC which can be used includes Bostex™ 561 (commerciallyavailable from Akron Dispersions, Akron, Ohio). The preferred thiazolecompound for use in the invention is zinc 2-mercaptobenzothiazole, alsoknown as zinc dimercaptobenzothiazole or ZMBT. Suitable ZMBT which canbe used includes Bostex™ 482A (commercially available from AkronDispersions, Akron, Ohio). In a preferred embodiment, the guanidinecompound used in the accelerator composition is diphenyl guanidine, alsoknown as DPG. Suitable DPG which can be used includes Bostex™ 417(commercially available from Akron Dispersions, Akron, Ohio).

[0032] Other dithiocarbamate, thiazole and guanidine derivatives canalso be use in accordance with the invention, provided each ischemically compatible with, i.e., does not substantially interfere withthe functionality of, the remaining two accelerator compounds used.Dithiocarbamate derivatives which can also be used include zincdimethyldithiocarbamate (ZMD), sodium dimethyldithiocarbamate (SMD),bismuth dimethyldithiocarbamate (BMD), calcium dimethyldithiocarbamate(CAMD), copper dimethyldithiocarbamate (CMD), leaddimethyldithiocarbamate (LMD), selenium dimethyldithiocarbamate (SEMD),sodium diethyldithiocarbamate (SDC), ammonium diethyldithiocarbamate(ADC), copper diethyldithiocarbamate (CDC), lead diethyldithiocarbamate(LDC), selenium diethyldithiocarbamate (SEDC), telluriumdiethyldithiocarbamate (TEDC), zinc dibutyldithiocarbamate (ZBUD),sodium dibutyldithiocarbamate (SBUD), dibutyl ammoniumdibutyldithiocarbamate (DBUD), zinc dibenzyldithiocarbamate (ZBD), zincmethylphenyl dithiocarbamate (ZMPD), zinc ethylphenyl dithiocarbamate(ZEPD), zinc pentamethylene dithiocarbamate (ZPD), calciumpentamethylene dithiocarbamate (CDPD), lead pentamethylenedithiocarbamate (LPD), sodium pentamethylene dithiocarbamate (SPD),piperidine pentamethylene dithiocarbamate (PPD), and zinc lopetidenedithiocarbamate (ZLD).

[0033] Other thiazole derivatives which can be used include2-mercaptobenzothiazole (MBT), copper dimercaptobenzothiazole (CMBT),benzthiazyl disulphide (MBTS), and 2-(2′,4′-dinitrophenylthio)benzthiazole (DMBT).

[0034] Other guanidine derivatives which can be used include diphenylguanidine acetate (DPGA), diphenyl guanidine oxalate (DPGO), diphenylguanidine phthalate (DPGP), di-o-tolyl guanidine (DOTG), phenyl-o-tolylguanidine (POTG), and triphenyl guanidine (TPG).

[0035] The proportions and ratios of the ingredients of the acceleratorcomposition can vary somewhat provided all three of the ingredients,i.e., dithiocarbamate, thiazole and guanidine compounds, are present.With respect to the preferred accelerator ingredients, each of theaccelerator compounds zinc diethyldithiocarbamate (ZDEC), zinc2-mercaptobenzothiazole (ZMBT) and diphenyl guanidine (DPG) can bepresent in an individual amount ranging from about 0.50 phr (parts byweight per 100 parts by weight of rubber) to about 1.00 phr dry weightper 100 parts polyisoprene. In other words, the accelerator compositionsof the invention comprise ZDEC:ZMBT:DPG phr dry weight ratios rangingrespectively from about 0.50:0.50:0.50 phr to about 1.00:1.00:1.00 phr.

[0036] In a preferred embodiment, a stabilizer is used in conjunctionwith the accelerator composition. Any stabilizer known in the art usefulin curable latex systems can be used provided it is chemicallycompatible with the other ingredients and provides the desired function,i.e., prolongs stabilization of the pre-cure compounded polyisoprenelatex. A variety of stabilizers can be used, including but not limitedto, milk protein salts, anionic surfactants such as sodium laurylsulfates, and sorbitan fatty acid esters.

[0037] Milk protein salts are preferred for use as the stabilizer. Inparticular, alkali earth metal caseinate salts are preferred. Alkaliearth metal caseinate salts which can be used in accordance with theinvention include, but are not limited to, sodium caseinate, potassiumcaseinate, manganese caseinate and zinc caseinate, and combinationsthereof. Most preferred for use as the stabilizer is sodium caseinate(commercially available from Technical Industries, Inc., Peacedale,R.I.).

[0038] Anionic surfactants which can be used as stabilizers for theinvention include Rhodopex® ES (a composition having a sodium lauryl (3)sulfate active available from Rhodia, Cranbury, N.J.) and Rhodacal®DS-10 (a composition having a branched sodium dodecylbenzene activeavailable from Rhodia, Cranbury, N.J.). Sorbitan fatty acid estersurfactants which can be used as stabilizers in the invention includepolyoxyethylene sorbitan fatty acid esters such as Tween® 80 (apolysorbate available from ICI Americas, Inc., Wilmington, Del.).

[0039] The amount of stabilizer present in the procure polyisoprenelatex composition is preferably ranges from about 0.50 phr dry weight toabout 1.00 phr dry weight (per 100.00 parts dry weight polyisoprene).Preferably, the amount of stabilizer is present in an amount of about0.75 phr dry weight.

[0040] In addition to the polyisoprene, accelerator composition andstabilizer, additional ingredients which enhance or facilitate themanufacturing process can be included in the compounded polyisoprenelatex composition as well. The compounded polyisoprene latex compositioncan also include catalysts (or accelerator initiators) such as alkaliearth metal oxides and methyl oxides, preferably zinc oxide (ZnO)(commercially available from Maxxim Medical, Eaton, Ohio); curing (orcross-linking) agents such as elemental Sulfur (e.g., Bostex™ 378commercially available from Akron Dispersion, Akron, Ohio), organicsulfides or other sulfur donor compounds; and anti-oxidants, such asWingstay™ L (e.g., butylated reaction product of p-cresol anddicyclopentadiene (DCPD) such as Bostex™ 24 available from AkronDispersion, Akron, Ohio).

[0041] Preparation of Polyisoprene Latex Composition

[0042] The compounded polyisoprene latex composition in accordance withthe invention can be prepared using the following general procedure:

[0043] Polyisoprene latex (typically 60% solids) and the stabilizer(e.g., sodium caseinate) are combined at ambient temperature (about 20°to about 25° C.). After mixing for a period of time, the mixture is thendiluted to 40% solids in water. Wingstay L is then added and the mixtureis stirred for approximately 15 minutes. At this point, the pH can beadjusted to a range of about 8.5 to 9.0. Zinc oxide is added, followedby the sulfur and accelerator compounds. Preferred accelerator compoundsare ZDEC, ZMBT and DPG and are added in ratios ranging from0.50:0.50:0.50 phr to 1.00:1.00:1.00 phr dry weight per 100.0 partspolyisoprene. The mixture is then heated to a temperature within a rangeof about 20° C. to about 40° C., preferably from about 25° C. to about30° C., while continuously stirring for a time period ranging from about1.5 hours to about 2.5 hours, preferably about 2 hours, using a magneticstirrer and heating plate.

[0044] The mixture is then cooled to a temperature ranging of less thanabout 25° C., typically ranging from about 15° C. to about 20° C. Thecompounded latex is preferably stored at ambient temperatures rangingfrom about 15° to about 20° C. At these temperatures, the compoundedpolyisoprene latex composition can be stored for periods lasting up toabout 8 days prior to its use in the dipping and curing process.

[0045] Preparation of a Polyisoprene Glove

[0046] Initially, the pH of the compounded polyisoprene latex can beadjusted to a pH of approximately 10. A glove former is pre-heated in anoven to a temperature of about 70° C. and then dipped in a pre-preparedcoagulant composition at a temperature of about 55° C. for a period oftime and then removed therefrom. Next, the coagulant-coated former isplaced in a drying oven at 70° C. for a time sufficient to dry thecoagulant, typically about 5 minutes.

[0047] The coagulant-coated former is removed from the oven and dippedinto the compounded polyisoprene latex at ambient temperature, or atemperature ranging from about 20° C. to about 25° C. The coated formeris removed and placed in oven at a temperature of about 70° C. for about1 minute. The glove and former are removed from oven and placed intowater leaching tank having a temperature of about 65° C. for about 5minutes. The glove and former are removed from the leaching tank andplaced dried at about 70° C. for a period sufficient to dry the glove,typically about 5 minutes. This is the end of the first curing stage.

[0048] At the second curing stage, the glove and former are placed in anoven heated to a temperature of about 120° C. for about 20 minutes. Theglove and former are removed and cooled to ambient temperature. Finally,the glove is stripped from the former.

[0049] The gloves can be further treated in accordance with theparticular needs, such as using lubrication, coating, halogenation, andsterilization techniques, all of which are conventional. Otherconventional steps can be incorporated into the general process as well.

[0050] When prepared in accordance with the invention, elastomericarticles such as gloves exhibit the following physical properties:tensile strength of greater than about 3000 psi, elongation of greaterthan about 750% at break, and a tensile modulus of less than about 300psi at 300% elongation as measured in accordance with ASTM D412.

[0051] Other elastomeric polyisoprene articles can be prepared usingprocesses similar to those described herein, in combination withconventional equipment and techniques readily available in the art. Forexample, an elastomeric article in the form of condom can be preparedusing a condom former.

[0052] The following example further illustrates the advantages of theinvention and should not be construed as limiting the invention to theembodiments depicted therein.

EXAMPLES Example 1 Preparation of a Polyisoprene Glove

[0053] Polyisoprene latex (Kraton™ IR PR401 lot #000313 having TSC64.40% obtained from Shell International Corporation, Houston, Tex.) wasdiluted with water. Sodium caseinate (obtained from TechnicalIndustries, Inc., Peacedale, R.I.) was then added to the mixture andstirred at ambient temperature. While under continuous stirring, zincoxide and sulfur dispersions were added to the mixture. Acceleratorcompounds ZDEC (from Akron Dispersions, Akron Ohio), ZMBT, and DPG (fromAkron Dispersions, Akron, Ohio) were formulated into dispersions andthen added. Wingstay™ L was added and the mixture was stirred forapproximately 15 minutes. The composition was diluted to about 37.0%solids with water. The pH was adjusted using ammonium hydroxide to pH10.7. The composition was maintained at a temperature of 25° C. andstored under continuous agitation for 24 hours at a temperature of lessthan 25° C.

[0054] Accordingly, the following is a summary of the formulationingredients and their respective amounts. All percentages arepercentages by weight unless otherwise noted. Latex Formulation:Ingredient Parts (phr) dry weight Polyisoprene 100.00 ZDEC 0.50 ZMBT0.50 DPG 1.00 Sodium caseinate 0.75 ZnO 0.50 Sulfur 1.25 Wingstay ™ L2.00

[0055] A glove former was preheated to 100° C. in an oven, removed anddipped into a coagulant composed of soft water 80.65%, calcium nitrate13.65%, calcium carbonate 5.46%, wetting agent (Surfonyl™ TG 0.2%),cellulose (Cellosize™ QP 52000) 0.04%) at a temperature of 56° C. for aperiod of 30 seconds and removed. The coagulant-coated former was cooledto a temperature of about 58° C. and was placed in a drying oven at atemperature of 100° C. for a period of time sufficient to dry thecoagulant.

[0056] The coagulant-coated former was removed from the oven and dippedinto the compounded polyisoprene latex composition of Formula 1 at atemperature of 25° C. for a period of 0.8 minutes. The coated former wasremoved and placed into a pre-heated oven at a temperature of 130° C.for a period of 0.8 minutes.

[0057] The coated former was then removed from the oven and placed intowater leaching tank at a temperature of 50° C. for a period of 5.0minutes. The former was removed from the leaching tank and placed intoan oven at a temperature of 70° C. for 30 seconds.

[0058] The former was removed from the oven and dipped into a siliconetank at a temperature of 40° C. for 30 seconds. The former was removedfrom the silicon tank and while still on the former, the glove wasbeaded at the cuff using a beader roller.

[0059] The former were then placed into a second stage cure oven andmoved therethrough at zone temperatures ranging from 110° C. to 135° C.for a total time period lasting for a period of 9.5 minutes. Afterexiting the curing oven, the glove was subjected to a post-cureleaching. At this step, the glove on the former was rinsed with water ata temperature of 70° C. water for a period of about 1 minute.

[0060] The glove was placed in a slurry tank at a temperature of 55° C.for 30 seconds. The slurry composition contained 85.2% water, 14.33%starch, 0.4% cellulose (Cellosize™ QP 52000), 0.4% sodium hypochlorite,0.01% surfactant (Darvan™), and 0.02% Casastab™ T. The formers were thenplaced into a post-slurry oven to dry the glove thereby producing thefinal glove. The glove covered former was cooled and the glove wasstripped therefrom.

[0061] The physical properties of the glove produced by the aboveprocess were evaluated. Samples were obtained from the gloves exhibitedaverage tensile strength values of 3810 psi, tensile modulus value of171 psi at 300% elongation, and 1125% elongation at break as measuredusing ASTM D142.

Example 2 Comparative Data using Different Accelerator Formulations andProcess Conditions

[0062] Differing compounded polyisoprene latex compositions and varyingprocess parameters were used to prepare samples, the physical propertiesof which were then tested and evaluated. Compounded latex containingvarious accelerator compounds and phr (parts per hundred) ratios wereprepared in accordance with a process similar to that of Example 1Process for Preparation of Polyisoprene Latex Composition”, andpre-cured and stored at the corresponding temperatures and conditionsdescribed or listed in Table 1 below.

[0063] Test samples were prepared from compounded latex formulations atvarious intervals over a total latex storage period of eight (8) days.Each of samples 1 and 3 through 16 were then prepared by heating platesto a temperature of about 70° C. for a period of about 5 minutes, andsubsequently dipping the plates in coagulant (35% calcium nitrate, 7%calcium carbonate, 0.03% Surfonyl# TG) at a temperature of about 55° C.for a period of about 10 seconds. The coagulant coated plates were thendried at 70° C. for a period of about 5 minutes. The coated plates werethen dipped into the compounded polyisoprene compositions, which werestored and dipped at the corresponding temperature shown in Table 1. Theplates were leached with water at a temperature of about 65° C. for aperiod of about 3 minutes, and subsequently dried at a temperature ofabout 70° C. for a period of about 5 minutes. The plates were then curedat a temperature of 120° C. for a period of about 20 minutes. Thesamples were then stripped from the plates.

[0064] Samples 2a and 2b were prepared using slightly different processparameters and were obtained from articles prepared usingmanufacturing-scale parameters and equipment. For each of samples 2a and2b, a mold (glove former) was heated to a temperature of about 55° C.and dipped in coagulant (same coagulant as above) at a temperature ofabout 55° C. The coagulant-covered mold was then dried in an oven at atemperature of about 70° C. for a period of about 3 minutes. The driedcoagulant-coated mold was removed from the oven and dipped into thecompounded latex composition for a period of about 12 seconds dwellingtime, removed for a period of about 6 seconds unsubmerged, and thenredipped for a further 8 seconds. The latex-coated mold was leached attemperature of about 50° C. for a period of about 5 minutes, andsubsequently cured at a temperature of about 135° C. for a period ofabout 15 minutes.

[0065] The following Table I is a summary of the process parameters andcompounded latex formulations prepared: TABLE 1 Accelerator andStabilizer Formulations and Process Conditions Accelerator Composition(ZDEC/ ZMBT/DPG Stablizer Sample No. Pbr ratio) (type/phr)Storage/Dipping Sample 1  1.0/1.0/0.50 Na Caseinate/0.75 ambient/ambientSample 2a 0.50/1.0/1.0 Na Caseinate/0.75 20-25° C./ambient Sample 2b0.50/0.50/1.0 Na Caseinate/0.75 20-25° C./ambient Sample 3  1.0/1.0/0.50Na Caseinate/0.75 16-18° C./ambient Sample 4*  1.9/0/0.50 NaCaseinate/0.75 ambient/ambient Sample 5   0/2.1/0.50 Na Caseinate/0.75ambient/ambient Sample 6  1.0/1.0/0 Na Caseinate/0.75 ambient/ambientSample 7  1.9/0/0 Na Caseinate/0.75 ambient/ambient Sample 8 1.0/0.50/0.25 Na Caseinate/0.75 ambient/ambient Sample 9  1.0/1.0/0.50DS10/0.75 ambient/ambient Sample 10  1.0/1.0/0.50 ES/0.3 ambient/ambientSample 11  1.0/1.0/0.50 Tween ® 80/0.75 ambient/ambient Sample 12** 1.0/1.0/0.50 Na Caseinate/0.75 ambient/ambient Sample 13*** 1.0/1.0/0.50 Na Caseinate/0.75 ambient/ambient Sample 14**** 1.0/1.0/0.50 Na Caseinate/0.75 ambient/ambient Sample 15  1.0/1.0/0.50Na Caseinate/0.75 16-18° C./ambient Sample 16  1.0/1.0/0.50 NaCaseinate/0.75 16-18° C./ambient

[0066] DS10 refers to Rhodacal® DS-10 which comprises sodiumdodecylbenzene (branched) available from Rhone-Poulenc, Inc., Dayton,N.J. ES refers to Rhodapex ES which comprises sodium lauryl (3) sulfateavailable from Rhone-Poulenc, Inc., Dayton, N.J. Tween® 80 comprisespolysorbate 80 and polyoxyethylene (20) sorbitan monooleate availablefrom ICI Americas, Inc. (Wilmington, Del.). Unless indicated otherwise,“ambient” temperature was measured as approximately 20° C. Precuretemperature and time for each of Samples 1 through 11 was a temperatureof 30° C. for a period of approximately 2 hours (120 minutes).

[0067] Each of the samples was then evaluated for tensile strength inaccordance with ASTM D 412-98a “Standard Test Methods for VulcanizedRubber and Thermoplastic Elastomers—Tension” (1998) with no exceptionsusing an Instron® testing apparatus. The average tensile strength valuesfor each sample were calculated from averaging five individual samplesper day storage value. The average tensile strength values for each ofthe samples tested are summarized in the following Table 2: TABLE 2Tensile Strength Corresponding to Differing Latex Storage PeriodsTensile Strength (psi) @ Compounded Latex Storage Time Sample # Day 0Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 Day 8  1 2698 3648 3398 3080 —2651 2548 — 2410  2a — 3768 3640 3839 — — 3441 3541 —  2b — 3498 37823882 — — 3939 3043 —  3 2201 2413 3192 3158 3288 3154 3008 3000 2909  43242 3609 — 3515 3244 3096 2498 — 2464  5 1483 1733 — 2149 1590 15341478 — 1358  6 No tensile measured/sample did not break  7 No tensilemeasured/sample did not break  8 1018 1063 3051 — 2177 — — 1802 —  9 —2566  914 2843 — — — — — 10 — 1278 2520  839 — — — — — 11 — 2407 29013042 2834 — — — — 12 — 2450 — — — 2374 — 2212 — 13 2544 3213 3181 29742770 — 2393 — — 14 1595 2221 2838 — 2383 — — 1805 — 15 2084 2974 24523497 3312 3075 3056 2979 2968 16 2194 2904 3064 3110 3170 3002 2885 29022746

[0068] As can be seen from the above data, synthetic elastomericpolyisoprene samples prepared in accordance with the invention canexhibit significantly elevated tensile strengths of about 3000 psi, evenafter using compounded latex which has been stored for periods of atleast 5 days and lasting up to about seven (7) days. In general, thebest tensile strength values per day latex storage were obtained usingthe combination of the three preferred accelerator compounds(ZDEC/ZMBT/DPG) and preferred phr ratios (0.50 to 1.00/0.50 to 1.00/0.50to 1.00 phr), as well as the preferred stabilizer, sodium caseinate.Samples prepared without one of the three preferred acceleratorcompounds exhibited significantly lower tensile strength values, as canbe seen from Samples 4, 5, 6 and 7. Based on the results of testing ofsamples 6 and 7, the tensile strength values for these samples failed tomeet minimum FDA regulatory standards required for elastomeric materialsto be used for surgeon's gloves, which is set at about 2479 psi.

[0069] Samples 3, 15 and 16 were prepared from compounded latexcomprising the preferred accelerator composition ZDEC/ZMBT/DPG in a1.0/1.0/0.50 phr ratio, with sodium case mate as the stabilizer, andstored at temperatures ranging from about 16° C. to about 18° C. as seenin Table 1. As can be seen from Table 2, these samples exhibited thehighest combination of storage longevity in relation to high tensilestrength values.

[0070] Sample 4, which was prepared from an accelerator compositionwithout the thiazole compound, demonstrated high tensile strengthvalues. This compounded latex, however, exhibited an undesirable amountof precipitation of solids out of the composition.

[0071] Based on the tensile data that was compiled for samples 9, 10 and11, the use of stabilizers in the compounded latex other than preferredstabilizer sodium case mate resulted in samples with significantlyreduced tensile strength per given storage period when compared tosamples 1 through 3 and 15 and 16, for example.

[0072] Samples 12, 13 and 14 were prepared from latex compositions undervarying pre-cure time and temperature parameters. As can be seen fromTable 2, deviations in pre-cure temperature and time conditions can alsosignificantly effect the physical properties of the resulting materialas well.

Industrial Applicability

[0073] The invention is useful in manufacturing process for elastomericarticles composed of polyisoprene. The invention affords the ability toproduce synthetic polyisoprene articles which closely mimic the physicalproperties of elastomeric articles made from natural rubber latex. Theinvention can be advantageously incorporated into the manufacturing ofsurgical gloves, condoms, probe covers, dental dams, finger cots,catheters, and the like.

[0074] The invention has been described with reference to variousspecific and preferred embodiments and techniques. However, it should beunderstood that many variations and modifications can be made whileremaining within the spirit or scope of the invention as defined by theclaims set forth below.

What is claimed is:
 1. A process of making an elastomeric polyisoprenearticle comprising the steps of: a) preparing a compounded latexcomposition containing an accelerator composition and a stabilizer, saidaccelerator composition comprising a dithiocarbamate, a thiazole and aguanidine compound; b) dipping a former into said compounded latexcomposition; and c) curing said compounded latex composition on saidformer to form said elastomeric polyisoprene article.
 2. The process ofclaim 1, wherein said elastomeric polyisoprene article is a glove. 3.The process of claim 1, wherein said elastomeric polyisoprene article isa condom.
 4. The process of claim 1, wherein said elastomericpolyisoprene article is a probe cover.
 5. The process of claim 1,wherein said elastomeric polyisoprene article is a catheter.
 6. Theprocess of claim 1, wherein said accelerator composition comprises: zincdiethyldithiocarbamate; zinc 2-mercaptobenzothiazole; and diphenylguanidine.
 7. The process of claim 1, wherein said stabilizer comprisesa milk protein salt.
 8. The process of claim 7, wherein said stabilizercomprises sodium caseinate.
 9. The process of claim 1, wherein saidaccelerator composition comprises: a dithiocarbamate to thiazole toguanidine phr ratio of from about 0.50 phr to about 1.00 phrdithiocarbamate, from about 0.50 phr to about 1.00 phr thiazole, fromabout 0.50 phr to about 1.00 phr guanidine, per 100.0 phr polyisopreneof the compounded latex composition.
 10. A synthetic elastomericpolyisoprene article having a tensile of greater than about 3000 psi asmeasured in accordance with ASTM D412, said article being prepared by aprocess comprising the steps of: a) preparing a compounded latexcomposition containing an accelerator composition and a stabilizer, saidaccelerator-composition comprising a dithiocarbamate, a thiazole and aguanidine compound, and a stabilizer; b) dipping a former into saidcompounded latex composition; and c) curing said compounded latexcomposition on said former.
 11. The article of claim 10, wherein thearticle is a glove.
 12. The article of claim 10, wherein the article isa condom.
 13. The article of claim 10, wherein the article is a probecover.
 14. The article of claim 10 wherein the article is a catheter.15. The article of claim 10, wherein said accelerator compositioncomprises: zinc diethyldithiocarbamate; zinc 2-mercaptobenzothiazole;and diphenyl guanidine.
 16. The article of claim 10, wherein saidstabilizer comprises a milk protein salt.
 17. The article of claim 16,wherein said stabilizer comprises sodium caseinate.
 18. The article ofclaim 10, wherein said accelerator composition comprises: adithiocarbamate to thiazole to guanidine phr ratio of from about 0.50phr to about 1.00 phr dithiocarbamate, from about 0.50 phr to about 1.00phr thiazole, and from about 0.50 phr to about 1.00 phr guanidine, per100.0 phr polyisoprene of the compounded latex composition.
 19. Apolyisoprene latex composition comprising: a dithiocarbamate compound; athiazole compound; a guanidine compound; and a stabilizer.
 20. The latexcomposition of claim 19 wherein the latex composition comprises: zincdiethyldithiocarbamate; zinc 2-mercaptobenzothiazole; diphenylguanidine; and sodium caseinate.
 21. An accelerator composition for usein a process for making elastomeric polyisoprene articles, saidaccelerator composition consisting essentially of: a dithiocarbamatecompound; a thiazole compound; and a guanidine compound; wherein the phrdry weight ratio of each of the dithiocarbamate, thiazole and guanidineranges from about 0.50 to about 1.00 per 100.0 parts polyisoprene.
 22. Aglove composed of polyisoprene and having a tensile strength of greaterthan 3000 psi as measured in accordance with ASTM D412, said glove beingprepared from a polyisoprene latex composition comprising adithiocarbamate compound, a thiazole compound, and a guanidine compound.23. The glove of claim 18, wherein said polyisoprene latex compositionfurther comprises sodium caseinate.
 24. The glove of claim 19, whereinsaid latex composition is stored for up to about 7 days.